Conventionally, for the purpose of reducing the weight of automobile bodies, high-strength steel sheets have been often used for suspension parts or structural parts of automobile bodies. Suspension parts of automobiles are required to have fatigue characteristics of a notch-free material and notch fatigue characteristics, but there has been a problem in that conventional high-strength steel sheets are insufficient in such performance and do not allow parts to be reduced in sheet thickness.
To improve fatigue characteristics of a notch-free material, it is effective to make the structure finer. For example, Patent Literatures 1 and 2 describe hot-rolled steel sheets that contain ultrafine ferrite grains with an average grain size of less than 2 μm as hot-rolled, and contain bainite or the like as a secondary phase. It is described that these steel sheets are excellent in ductility, toughness, fatigue strength, and the like, and have low anisotropy in these characteristics. In addition, since a fatigue crack occurs from near the surface, it is also effective to make the structure near the surface finer. Patent Literature 3 describes a hot-rolled steel sheet having a crystal grain size gradient structure in which the average crystal grain size of polygonal ferrite, which is the main phase, gradually decreases from the center of the sheet thickness toward the surface layer and containing, in volume fraction, 5% or more bainite or the like as a secondary phase. Furthermore, grain refining of a martensite structure is also effective in improving fatigue characteristics. Patent Literature 4 describes a machine structural steel pipe in which 80% or more in area fraction of the microstructure is martensite, the average block diameter of the martensitic structure is 3 μm or less, and the maximum block diameter is 1 to 3 times the average block diameter. Patent Literature 4 also describes making the structure of a slab before pipe-making into lower bainite or martensite in hot rolling to uniformly disperse carbon. However, although grain refining improves fatigue characteristics of a notch-free material, it has no effect of delaying a crack propagation rate, thus not contributing to an improvement in notch fatigue characteristics.
Regarding an improvement in notch fatigue characteristics, it has been reported that it is effective to reduce a crack propagation rate by forming a composite structure. In Patent Literature 5, hard bainite or martensite is dispersed in a structure whose main phase is fine ferrite; thus, fatigue characteristics of a notch-free material and notch fatigue characteristics are both achieved. It is reported in Patent Literatures 6 and 7 that increasing an aspect ratio of martensite in a composite structure reduces a crack propagation rate.